1. Field of the Invention
The invention relates generally to solid-state light-emitting element drive devices, lighting systems and lighting fixtures and, more particularly, to a solid-state light-emitting element drive device that drives a solid-state light-emitting element, such as a light-emitting diode or an organic electroluminescence (EL) element, to emit light, and a lighting system and a lighting fixture that use the drive device.
2. Description of the Related Art
In recent years, a lighting system and a lighting fixture have rapidly become widely used, which adopts, as a light source, a solid-state light emitting element such as a light-emitting diode or an organic electroluminescence (EL) element, as substitute for an incandescent lamp and a fluorescent lamp. For example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-511078 discloses an LED drive device that adopts, as a light source, a light-emitting diode (LED) and adjusts (dims) amount of light outputted from the LED by increasing or decreasing output of a switching source circuit (a step-down chopper circuit) in response to a dimming signal provided by a dimmer.
Here, as a dimming method of an LED, there are a dimming method in which a magnitude of current continuously flowing to the LED is changed (hereinafter, referred to as a DC (Direct Current) Dimming Method), a dimming method in which a ratio of a conducting period (a duty cycle) is changed by periodically switching on and off the current flowing to an LED (hereinafter, referred to as a Burst Dimming Method), and the like. The latter Burst Dimming Method is adopted in the conventional LED drive device described in the above-mentioned document.
However, the conventional LED drive device that adopts the Burst Dimming Method has the problem that causes interference with video equipment, such as a video camera, thereby generating flicker. This is caused by a difference between a period of the burst dimming and a shutter speed (an exposure time) of the video equipment, and therefore, the flicker (variation in brightness) or streaky contrasting density appears on an image generated by the video equipment. In addition, a repetition frequency of a light output is required to be more than or equal to 500 Hz, according to enforcement of amendment to technical standards in Electrical Appliance and Material Safety Law (Japanese Laws) relating to an LED (standards in paragraph 1 of the Ministerial Ordinance that establishes technical standards in Electrical Appliances: Amendments of the Ministerial Ordinance on Jan. 13, 2012).
Incidentally, in a general step-down chopper circuit, when current flowing through an inductor reaches a threshold value during an ON-period of a switching element, the switching element is turned off at that timing, and then when a regenerative current reaches a lower limit (e.g., zero), the switching element is turned on again at that timing. Therefore, when a frequency of a burst signal is adapted to the above-mentioned technical standards, the following problem is generated with combination of the step-down chopper circuit and the drive device: even if the duty cycle of the burst signal is changed during an OFF-period of the switching element in the step-down chopper circuit, the inductor current does not change (See FIG. 5A), and as a result, it means that the light output of the LED changes along a tiered line with respect to a change in the duty cycle of the burst signal, as shown in FIG. 5B.
Here, a light output for each tier in FIG. 5B corresponds to a light output for a single period in the switching periods of the switching element. Therefore, if the switching periods of the switching element are shortened (if the switching frequency is increased), the light output for each tier reduces, thereby allowing the overall light output to change more linearly. However, increasing the switching frequency leads to an increase in the switching loss, and further when considering the performance of the drive circuit driving the switching element, making significantly higher frequency can't be expected.